This invention relates generally to a magnetic resonance imaging system (typically, nuclear magnetic resonance imaging system) for imaging a desired virtual cross-section of a desired portion of a subject to be inspected, utilizing a magnetic resonance phenomenon, and more particularly to a gradient magnetic field coil for use in such a magnetic resonance imaging system, the gradient magnetic field coil including a coil bobbin and coil windings fixed to the coil bobbin.
A known magnet portion 106 of a magnetic resonance imaging system as shown in FIG. 10 comprises a main magnetic field coil winding 101 for producing a Z-direction or Z-coordinate component of static magnetic field which generally constitutes a super conducting magnet (SCM), a cylindrical coil bobbin 102 arranged inwardly of the main magnetic field coil winding 101, and X-, Y- and Z-direction gradient magnetic field coil windings 103, 104 and 105 which are fixedly mounted on the coil bobbin 102 and modify the intensity of the Z-direction magnetic field along the X-, Y- and Z-directions, respectively, by producing a so-called gradient magnetic field. In the magnetic resonance imaging system, the dependence of the intensity of the magnetic fields (which are produced by the coil windings 101, 103, 104 and 105) on the position, that is, the dependence on the portion (position) of a subject to be inspected 107, is varied so as to vary that portion of the subject 107 from which a magnetic resonance signal is generated, thereby imaging an imaginary cross-section of any desired portion of the subject 107, for example, in view of a distribution of density of a specific isotope. At this time rectangular current pulses are applied to any one of the gradient magnetic field coil windings 103, 104 and 105 (hereinafter designated by 108 when they are collectively referred to), so that due to this pulse current, an electromechanical force of about oHe to several kHz is applied to that gradient magnetic field coil winding. As a result, particularly, the X- and Y-direction gradient magnetic field coil windings 108 which can not be securely fixed sufficiently to the coil bobbin 102 tend to vibrate to strike the coil bobbin 102, thus producing a large noise. For example, when the static magnetic field Ho produced by the main winding 101, the diameter of the coil bobbin 102 and the magnetic field gradient H.sub.GC were 0.5 tesla, 700 mm and 0.3 Gauss, respectively, the level of the noise at a position spaced 1 m from the magnet portion 106 is approximately 70 phon when noise reduction measure is not provided. With increase of the static magnetic field Ho and the magnetic field gradient H.sub.GC, the noise level increases abruptly, and when the static magnetic field Ho is 1.5 tesla, the noise level exceeds 100 phon.
It is known to interpose rubber between the gradient magnetic field coil windings 108 and the coil bobbin 102 in order to suppress the noise due to the above vibration (Such rubber is designated by a reference numeral 13 in FIGS. 1 to 4 of Japanese Patent Unexamined Publication No. 61-279238).
In the above Japanese Patent Unexamined Publication No. 61-279238, it is also proposed to fixedly mount a weight (designated by a reference numeral 14 in FIGS. 3 and 4 of this prior Japanese patent application) on the outer periphery of the coil windings 108 in order to reduce the proper or eigen frequency of the coil windings 108.
However, in the gradient magnetic field coil 109 having the coil bobbin 102 and the gradient magnetic field coil windings 103, 104 and 105, the provision of the rubber can hardly suppress the generation of the noise although the rubber is provided with an intension to suppress the transmission of the vibration from the coil windings 108 to the coil bobbin 102. Therefore, the subject 107 is annoyed with the noise.
On the other hand, the applicants have recognized that the provision of the rubber is not sufficiently effective because the generation of the noise is mainly due to a resonant vibration of the coil bobbin, and have come to the conclusion that it is necessary to suppress the vibration of the coil bobbin in such a manner as to rapidly attenuate the vibration, rather than to suppress the striking against the coil bobbin, because such striking is unavoidable to a certain degree.
It is known, for example, from NOISE/VIBRATION PREVENTION HANDBOOK (pages 319 to 321) published by Hankuhodo that when a partition wall 110 as shown in FIG. 11A is of a hollow structure having an air layer 113 between opposed side walls 111 and 112, a sound insulating effect against the sound propagating in a direction D can be obtained, and that when a partition wall 115 as shown in FIG. 11B has a collectively porous sound absorption material 114 (e.g. a mass of glass wool) filled in the hollow portion 113, the sound insulating effect is enhanced particularly in the range of frequencies of above 200 Hz. In these cases, each of the side walls 111 and 112 is composed of a flexible board 117 and a melamine resin plate 116 bonded thereto. The above HANDBOOK describes a graph indicating the dependence of the transmission loss on the frequency, this graph being shown in FIG. 12. However, the above HANDBOOK merely refers to general sound-insulating characteristics of the partition wall thus formed.
It is also known, for example, in a commercially available "vibration suppression steel plate" that when a thin plate-like body is of a so-called constrained damping structure having a viscoelastic layer between two parallel plates, vibration caused by an impact applied to one of the two parallel plates of this plate-like body can be rapidly attenuated.